Process of removing scale oxid from the surface of iron and steel.



UNITED STATES PATENT OFFICE.

WILLIAM FEENIE, OF BUHL, MINNESOTA, AND WILLIAM W. RHODES, OF WILMINGTONDELAWARE, ASSIGNORS T0 E. I. DU P01; '1 DE NEMOURS & COMPANY, OFWILMINGTON, DELAWARE, A CORPORATION OF DELAWARE.

PROCESS OF REMOVING SCALE OXID FROM THE SURFACE OF IRON AND STEEL.

No Drawing.

,moving the metal oxid which is usually present as a scale on thesurface of rolled or hammered iron or steel, and on the surface of alloysteels; and the invention pertams especially to the dissolving of suchscale with the aid of a solution of a metal bisulfate.

A widely used process for removm scale from iron and steel consisted insubJecting the iron or steel stock to the action of a water solutioncontaining sodium bisulfate and also, in some cases, stannous sulfate.Other methods of pickling iron or steel usually involved the use of anacid such as sulfuric acid. Although sulfuric acid was not as desirablea pickling agent as sodium bisulfate because of its dissolving action onthe metal itself, it was nevertheless frequently used instead of sodiumbisulfate on account of the lower cost of the process as compared withthe process using sodium bisulfate. In working with the latter it wasfound that the solution became inactive as far as its ability to removescale was con cerned after a certain percentage of the bi sulfate hadbeen used up, but while a considerable proportion of bisulfate remainedunchanged. Since it seemed impracticable to render all of the bisulfateavailable for ickling, it was looked upon unfavorably mm acost-standpoint in comparison with sulfuric acid, even though thepickled product showed less pitting and was better pickled than thesteel stock ickled with a solution of sulfuric acid. nly manufac-'turers who desired especially fine pickling regardless of cost couldafford to use bisulfate.

The principal object of our invention is to provide a process ofremoving scale oxid with a solution of sodium bisulfate wherebySpecification of Letters Patent. Patented Dec. 30, 1919. Applicationfiled April 3, 1919. Serial No. 287,361. A

a much larger proportion of the bisulfate in solution is renderedavailable than was the case in the old process.

Another object of our invention is to so modify the ordinary bisulfatepickling solu tion that it may be successfully used to remove the scalefrom alloy steels, such as nickel, chrome and vanadium steels.

The pickling action may be explained as follows:

The sodium acid sulfate attacks the ferrous oxid according to thefollowing equa- (310111.

For every molecule of ferrous oxid dissolved, two molecules of sodiumacid sulfate are used up in producing one molecule each of sodiumsulfate, ferrous sulfate and water.

In pickling with niter cake, which is commonly used for this purpose,there is present about 22% sodium sulfate in the cake, so that the freshbath contains sodium sulfate as such.

In the fresh bath sodium acid sulfate is highly dissociated, but as theamount of sodium acid sulfate diminishes due to its conversion to sodiumsulfate and ferrous sulfate and the concentration of the sodium sulfateand ferrous sulfate increases, the dissociation of sodium acid sulfatediminishes until it reaches practically zero, for there is noperceptible action on the iron or steel stock.

A pickling bath of niter cake which has become inactive maybe madeactive again in some degree by the addition of water,ibut theconcentration of the sodium bisulfate is thereby diminished and theresulting speed of pickling is too slow for commercial practice.

We have discovered that the activity of a bisulfate solution which hasbeen used for pickling until it has become almost inactive, may begreatly increased by decreasing the ratio of the normalsulfates to thebisulfates, and especially by removing sodium sulfate. The temperatureof the bath during pickling is between 100 and 200 F, and preferablyabout 140 F. We have found that a con-- venient method for decreasingthe ratio of normal'sulfates to bisulfates comprises reducing thetemperature by about 50 or 60 F.. When the solution is cooled to thisextent, for example from 140 F. to 85 F., a large proportion of thesodium sulfate and a smaller proportion of the ferrous sulfate containedin solution will crystallize and become segregated in a watery layernear the bottom of the tank, whereas all of the bisulfate present willremain in solution. The layer containing crystallized sodium sulfate isthen separated from the supernatent liquid; the latter will be found tohave regained by this treatment an actlvlty or speed approaching'that ofa fresh solution containin the same amount of sodium bisulfate. Theexpression speed of pickling is used to denote a comparison of theelapsed time the steel or iron stock requires to pickle compared withthe standard elapsed time for pickling in a fresh. solution.

very fair rate of speed, but not maximum speed because with maximumspeed the eliiciency is somewhat sacrificed.

-We shall take for. example as stock to be pickled ironwire varying from10 gage B. & S. to *3" rods.

A pickling tank of approximately 160 cubic feet capacit is used so thatwhen filled to the working evel it will contain 1000 gallons of water.The tank is filled with 1000 gallons of water and 1500 lbs. ofniter cakeadded, and the niter cake dissolved. The temperature of the solution isbrought up to approximately 140 F. and maintained at that temperature asnearly as possible.

. There is now added a rack of the stock containing a net weight of 2400lbs. of wire and this stock is allowed to pickle one-half hour, when thescale and oxid will have been removed. This rack of stock is now removedandanother added and allowed to pickle for one-half hour; this procedureis continued until a total of four racks or 9600 lbs. of wire have beenpickled. If, at this point, no further niter cake were added to thesolution, the speed of pickling would rapidly diminish; 2 00 lbs. ofniter cake is added and allowed to dissolve. Two more racks are thenpickled, allowing each to remain submerged one-half hour. The totalquantity now pickled is 14,400 lbs. of wire.

300 lbs. of niter cake are again added and two more racks are pickled sothat the total becomes 19,200 lbs. of wire pickled.

Three hundred more pounds of cake are then added and two more rackspickled. There has now been pickled 24,000 lbs. of wire and a total of2400 lbs. of niter cake has been added to the bath.

The bath is now near its saturation point for-salts, viz., 21} lbs.pergallon, and if the pickling were continued it would soon diminish inspeed and become inactive, al-

though there is still sodium acid sulfate tallizes and separates as awatery layer near the bottom of the tank and mixed with it is usually acertain amount of crystallized ferrous sulfate. The mechanically removedsludge is also on the bottom of the tank.

25% of the cubical contents of the tank lying near the bottom is thenslowly drained 0E through a suitable outlet. This removes the mechanicalsludge, a great portion of the sodium sulfate and some of the ferroussulfate.-

The tank in nowmade up to its original level with water and 25% of theoriginal charge of niter cake added, viz., 375 lbs. The temperature isnow raised to 140 F., and exactly the same pickling procedure down tothe draw off as outlined above is followed, so that 4.8,000-lbs. of wireis completed as a total to this point.

Another draw off is resorted to and the schedule repeated and the totalwire pickled becomes72,000 lbs. Still another draw off may be made andthe schedule again repeated until a grand total of 96,000 lbs. of

wire has been pickled.

As stated above, it is not possible to eliminate the ferrous sulfateentirely by the draw 011' method, and at this point the ferrous sulfatebecomes very concentrated and the solution has gradually assumed agreenish appearance. Nothing can now be done to maintain its speed andthe tank is consequently emptied of its entire contents.

The amount of steel pickled per unit of acidity by this new bisulfateprocess is greater than for a solution of sulfuric acid, due to the factthat we-have established a procedure wherein the acid content of thesodium bisulfate or niter cake has been made effective and also due tothe fact that sodium bisulfate attacks only the oxid and has littleaction on the iron after it becomes bared of its oxid coating.

7 The greatly increased efliciency of this new process over the oldprocess is easily seen, for in the old process to maintain fair speed itwas only possible to pickle ten pounds of wire per pound of niter cakeused, whereas in the present process 15.4 pounds of wire may be pickledper pound of niter cake used; i. e., a 50% greater yield of pickledproduct from-the same qu-antityof We claim:

1. The process of removing scale oxid from iron and steels whichcomprises subjecting the scale-coated metal to the action of anunsaturated sodium bisulfate solution, maintained at a temperature offrom 100 to 200 F., thereby incidentally forming normal salts, addingfurther amounts of sodium bisulfate to said solution to enable saidsolution to exert a solvent action on further quantities of scale-coatedmetal, subjecting further quantities of metal to the resulting solution,continuing to make such additions until said solution becomes saturatedwith salts and no longer exerts a vigorous solvent action on scale, andthen decreasing the ratio of the normal salts to the sodium bisulfate torestore the scale-dissolving ac tivity of the bisulfate solution.

2. The process of removing scale oxid from iron and steels whichcomprises subjecting the scale-coated metal to the action of anunsaturated sodium bisulfate solution, maintained at a temperature offrom 100 to 200 F., thereby incidentally forming normal salts, addingfurther amounts of sodium bisulfate to said solution to enable saidsolution to exert a solvent action on further quantities of scale-coatedmetal, subjecting further quantities of metal to the resulting solution,continuing to make such additions until said solution becomes saturatedwith salts and no longer exerts a vigorous solvent action on scale, thenlowering thetemperature of the solution to precipitate at least part ofthe relatively insoluble normal salts contained therein, separating theprecipitate from the remaining bisulfate solution, and subjectingfurther quantities of scale-coated metal to the action of said remainingbisulfate solution.

3. The process of removing scale oxid from iron and steels whichcomprises subjecting the scale-coated metal to the action of anunsaturated sodium bisulfate solution, maintained at a temperature ofabout 140 F.. thereby incidentally forming normal salts, adding furtheramounts of sodium bisulfate to said solution to enable said solution toexerta solvent action on further quantities of scale-coated metal,subjectin further quantities of metal to the resulting solution,continuing to make such additions until said solution becomes saturatedwith salts and no longer exerts a vigorous solvent action on scale, thenlowering the temperature of the solution to precipitate at least part ofthe sodium sulfate contained therein, separating the precipitate fromthe remaining solution, and subjecting further quantities ofscale-coated metal to the action of said remaining solution at anelevated temperature.

4:. The process of removing scale oxid from iron and steels whichcomprises subjecting the scale-coated metal to the action of a saturatedsolution of sodium bisulfate at an elevated temperature until the normalsodium sulfate in solution has accumulated to such an extent as toeffectually weaken the solvent action of the remaining bisulfate, thenremoving at least a substantial part of said normal sodium sulfate whichis in solution to render said solution unsaturated with respect to saidnormal sulfate at said elevated temperature, and subjecting furtherquantities of scale-coated metal to the action ofthe resultingbisulfate-containing solution.

5. In the process of removing scale-oxid from iron and steels th step ofrestoring the scale-dissolving action of a sodium bisulfate solutionwhich has become inactive which comprises so decreasing the ratio ofnormal sulfate, which is present in said solution, to the bisulfate asto render said solution unsaturated at normal working temperature withrespect to sodium sulfate.

6. In the process of removing scale-oxid from iron and steels the stepsof restoring the scale-dissolving action of a sodium bisulfate solutionwhich has become inactive which compris so cooling the solution as toprecipitate normal sodium sulfate while avoiding precipitation ofsubstantial pro portions of sodium bisulfate, and separating theprecipitate from the remaining solution.

In testimony whereof we aifix our signatures.

WILLIAM FEENIE. WILLIAM W. RHODES.

